Question

我需要为SQLite转换以下表达式，知道如何做到这一点吗？

IF EXISTS (SELECT [ID] FROM [USERS] WHERE [ID]=1)
SELECT [ID], [NAME], [CITY] FROM [USERS] WHERE [ID]=1
ELSE   
SELECT NULL AS ID, NULL AS NAME, NULL AS CITY

UNION ALL

IF EXISTS (SELECT [ID] FROM [USERS] WHERE [ID]=2)
SELECT [ID], [NAME], [CITY] FROM [USERS] WHERE [ID]=2
ELSE   
SELECT NULL AS ID, NULL AS NAME, NULL AS CITY

换句话说，当条件不符合条件时，我需要返回一个带有NULL值的必要列的空行。

Answer 1

您可以创建包含所需ID的虚拟表格，然后使用左侧outer join获取匹配的Users记录值或NULL：

SELECT Users.*
FROM (SELECT 1 AS ID
      UNION ALL
      SELECT 2) AS IDList
LEFT JOIN Users ON IDList.ID = Users.ID

Answer 2

SQLite没有IF语句或任何其他控制流语句。

你可以做的是将实际记录（如果存在）和一起取出NULL记录，然后只取第一个记录：

SELECT * FROM (SELECT ID, Name, City FROM Users WHERE ID = 1
               UNION ALL
               SELECT NULL, NULL, NULL
               LIMIT 1)
UNION ALL
SELECT * FROM (SELECT ID, Name, City FROM Users WHERE ID = 2
               UNION ALL
               SELECT NULL, NULL, NULL
               LIMIT 1)

返回带有NULL值的必要列的空行

2 个答案: